Optical waveguide and fabrication method

ABSTRACT

An optical waveguide with at least a guiding lamina ( 10 ) of optical material bonded by direct interfacial bonding to a superstructure lamina ( 20 ) of optical material, in which regions of the guiding lamina have modified optical properties so as to define a light guiding path along the guiding lamina. In a particular example, a periodically poled LiNbO 3  planar waveguide is buried in LiTaO 3  by direct interfacial bonding and precision polishing techniques and used in an optical frequency doubling system.

This application is the US national phase of International ApplicationNo. PCT/GB99/03055, filed 14 Sep. 1999, which designated the U.S., theentire content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the fabrication of optical waveguides.

2. Discussion of Prior Art

One known technique for fabricating optical waveguides is the so-calleddirect bonding (or direct interfacial bonding) technique.

Direct bonding (DB) is a fabrication technique that uses the Van derWaals forces present when two atomically flat bodies approach each otherto create a bond between two bodies. If the bodies are laminas ofoptical material having appropriate refractive indices, the materiallaminas can be joined to form waveguiding boundaries.

In one established way to form such a bond the surfaces of two pieces ofoptical material are polished so as to be very flat (i.e. substantiallyflat at atomic dimensions). The crystalline structures of the twopolished faces are preferably aligned with each other and the polishedfaces are pressed together. A heat treatment can be useful to encouragea pyroelectric effect and the exchange of electrons between the twosurfaces. This gives rise to an electrostatic attraction between the twosurfaces, which tends to expel any remaining air or liquid from betweenthe two surfaces. A final annealing step can improve the bond strengthfurther.

A DB bond can be formed irrespective of the lattice constants andorientation of the bodies involved and causes no degradation on thecrystalline microstructure or either material. By contacting surfaces insuch a non-destructive way, DB preserves the bulk characteristics ofeach bonded material whilst avoiding possible problems caused by latticedefects, such as increased propagation loss and optical damage.

EP-0598395 describes forming an optical waveguide device by directbonding of a support substrate and a low refractive index layer on aglass substrate, then etching the glass substrate.

SUMMARY OF THE INVENTION

This invention provides an optical waveguide comprising at least aguiding lamina of optical material bonded by direct interfacial bondingto a superstructure lamina of optical material, in which regions of theguiding lamina have modified optical properties so as to define a lightguiding path along the guiding lamina characterised in that thewaveguide further comprises a second superstructure lamina bonded bydirect interfacial bonding to the guiding lamina.

The invention recognises and addresses the shortcomings of previousproposals for the use of DB structures in optical waveguides. In suchprevious proposals, a flat lamina of a material having a raisedrefractive index (forming a waveguide “core”) is bonded between twolaminas of material having a lower refractive index (forming a waveguide“superstructure”). While this provided a bulk guiding structure, thelarge lateral dimension of the flat “core” lamina meant that thearrangement was not useful for many waveguiding applications or as asingle-mode waveguide.

In contrast, in the invention, regions of the core lamina have modifiedoptical properties so as to define a light guiding path along the corelamina. This can give a greatly increased flexibility of use and allowthe guiding path to be much more tightly defined than in previousarrangements.

Although the method is suitable for use with many types of materials,such as glasses, it is preferred that the core lamina is a ferroelectricmaterial, allowing the modified regions to be generated by electricalpoling.

A particularly useful ferroelectric material having well-studied opticaland electrical properties, is periodically poled lithium niobate (PPLN).PPLN combines a large non-linear coefficient, a widely-controllablephase-matching wavelength, and zero walk-off characteristics that makeit an ideal material to achieve quasi-phase matching (QPM) fornon-linear frequency conversion. With recent improvements in theefficiency of second-harmonic generation (SHG) within PPLN substrates,it is recognised in the present invention that the use of such amaterial in an appropriate waveguide geometry formed using the inventioncan provide a realisation of various compact non-linear devices based onharmonic or parametric generation.

The present method is particularly appropriate for use with PPLN, andhas several advantages over other techniques for fabricating waveguidesusing PPLN such as the so-called “annealed proton exchange” techniqueand the “titanium indiffusion” technique, both of which act on a singlePPLN crystal and modify the crystal near the surface in order to createregions of higher refractive index for optical confinement.

Previous experiments investigating the bonding characteristics of PPLNhave been directed towards fabricating thick multi-laminated stacks ofthe material for a large physical aperture, and thus high powerapplications. In contrast, creating a sufficiently thin lamina of PPLNincreases the average pump intensity applied to the domain-invertedstructure via optical confinement, and thus allows efficient SHG even atlow pump powers. Fabrication of such a device is obtainable by bondingPPLN onto a suitable substrate before precision polishing down towaveguide dimensions, a method which has already been demonstrated inthe production of LiNbO₃ planar waveguides for electro-opticapplications. One of the primary attractions offered by this techniqueis that the non-linearity and domain characteristics of the PPLNstructure after bonding should remain unchanged from the bulk material—acombination that annealed proton exchange and Ti indiffusion methods areclose to achieving, but not yet at their full theoretical efficiencies.A further advantage of the present method is the extra flexibilityavailable when designing devices, as combinations of multiple laminaswith different material properties are now possible.

Viewed from a second aspect this invention provides a method offabricating an optical waveguide, the method comprising the steps of:

(a) bonding, by direct interfacial bonding, a guiding lamina of opticalmaterial to a superstructure lamina of optical material;

(b) before, during or after step (a), modifying optical properties ofregions of the guiding lamina so as to define a light guiding path alongthe guiding lamina; characterised in that the method further comprisesthe steps of:

(c) after steps (a) and (b), removing material from the guiding laminato reduce the thickness of the guiding lamina (10); and

(d) after step (c), bonding, by direct interfacial bonding, a furthersuperstructure lamina (20) to the guiding lamina.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the inventions will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a waveguide formed using a lamina ofPPLN bonded between two laminas of lithium tantalate;

FIG. 2 schematically illustrates a second harmonic generator using thewaveguide of FIG. 1;

FIG. 3 is a graph relating the square root of second harmonic power tolaunch power for the apparatus of FIG. 2;

FIGS. 4 and 5 are schematic diagrams illustrating the fabrication of awaveguide according to an embodiment of the invention using anindiffusion technique; and

FIG. 6 schematically illustrates a waveguide according to a furtherembodiment of the invention.

DETAILED DISCUSSION OF EMBODIMENTS

In the following description, preparation and use of an examplewaveguide as a second harmonic generator will first be described withreference to FIGS. 1 to 3. Then, other waveguides also formingembodiments of the invention will be described.

FIG. 1 schematically illustrates a waveguide formed as a directly bondedsandwich of a lamina 10 of PPLN between two laminas 20 of lithiumtantalate (LiTaO₃).

The PPLN lamina 10 is in the form of a PPLN grating, in that the lithiumniobate (LiNbO₃) material is poled in a periodic, “striped” arrangement.These “stripes” of alternately poled regions in the lithium niobatematerial are shown schematically in FIG. 1 as alternate black and whitestripes, although it will be appreciated that in reality the periodicpoling structure would almost certainly not be detectable by the humaneye.

Production of the PPLN grating began with a 0.5-mm-thick single domainz-cut LiNbO₃ sample of about 15 mm×15 mm surface area. A photoresistpattern was created on the z-face of the crystal by photolithography.This formed regions on the crystal surface which are covered by anelectrical insulator, and regions which are not so covered. A liquidelectrode was then applied to the partially insulated surface, anddomain inversion in the z-axis was performed at room temperature by theapplication of a single high voltage pulse of ˜11 kV through the liquidelectrode. This resulted in three 5.5-mm-long PPLN gratings, positionedin the centre of the LiNbO₃ sample at 1 mm intervals. Grating periods of6.58, 6.50, and 6.38 μm were created, the first two of which aresuitable for frequency doubling of a Nd:YAG laser operating at 1064 nm.

LiTaO₃ was chosen as a suitable material for both the substrate andsuperstructure laminas as it combines thermal characteristics that are agood match for LiNbO₃, an important pre-requisite when annealing bondsat high temperatures, together with a refractive index lower than thatof LiNbO₃.

Each LiTaO₃ substrate was 0.5-mm-thick and shaped relative to the PPLNsample to provide a bonding area of about 12 mm×10 mm between the twooptically flat surfaces. To form a bond between an LiTaO₃ substrate andthe PPLN grating, the two materials were first cleaned, then a mixtureof H₂O₂—NH₄OH—H₂O (1:1:6) was applied to both materials, followed byseveral minutes of rinsing in de-ionised water, in order to render theirsurfaces hydrophilic.

Contacting of the PPLN and LiTaO₃ laminas was performed at roomtemperature with both samples aligned along the same crystallineorientation. A heat treatment of 120° C. immediately followed crystalcontact to induce the pyroelectric effect at the DB interface. Theresultant electrostatic attraction forced any excess air or liquid frombetween the two surfaces, whilst bringing them close enough to encouragethe formation of hydrogen bonds. This effect was evident by theelimination of most contact fringes at the crystal interface. Annealingof the bonded sample at 320° C. for 6 hours provided a bond strengthsufficient for further machining, and the PPLN region was lapped down toobtain a waveguiding lamina of 12-μm-thickness.

The second superstructure lamina of LiTaO₃ was then added as above. Thefinal DB structure included bonded interfaces of about 12 mm×10 mm aboveand below the PPLN core, although evidence of small unbonded regions atthe edges of the sample were detected by the presence of opticalfringes. The unnecessary material surrounding the gratings was laterremoved using dicing equipment and the waveguide end-faces were thenpolished to a parallel optical finish. Dimensions of the resultingburied PPLN planar structure are given schematically in FIG. 1.

An upper limit for the value of the propagation loss of the waveguidestructure was found by measuring the transmission of a 1064 nm laserbeam when end-launched into the waveguide. It was noted that thetransmission changed between the PPLN and unpoled LiNbO₃ sections,although this was not due to SHG. The launch from a microscope objectivewas empirically optimised for each region and maximum transmissions of81% were found at the edges of the poled regions (where the best SHGoccurred) and throughout the unpoled LiNbO₃ sections, whilst 65%transmission was obtained at the centre of the PPLN region. Thus, takinginto account the 5.5-mm-length of the grating, an upper-limit to thepropagation loss in each section can be placed as 1.7 dB cm⁻¹ for thePPLN edges and unpoled LiNbO₃ regions, and 3.4 dB cm⁻¹ for the centralPPLN region. In reality, these transmission figures also include acertain loss due to non-perfect launching and so the propagation lossesare likely to be much lower. Indeed, DB waveguides in garnets andglasses for laser applications have shown losses of ˜0.5 dB cm⁻¹ andless.

To test the non-linear properties of the buried PPLN structure, the SHGcharacteristics of the 6.50 μm grating were investigated. This grating,which occupied the middle section of the PPLN waveguide, successfullysuppressed the photorefractive effect at its phase-matching temperatureof 174.1° C. and so was chosen for further analysis. The 1064 nm pumpsource was a cw diode-pumped Nd:YAG laser 30 operating with multi-axialmodes. The linear polarisation state was rotated with a half-wave plate(not illustrated) to be parallel with the z-axis of the PPLN in order toaccess the material's largest non-linear coefficient (d₃₃). Focusing ofthe pump radiation for launching into the waveguide was performed usinga combination of microscope objectives and cylindrical lenses, as shownin FIG. 2. In particular, the initially circular pump beam was passedthrough a spherical collimating lens 40 and into a ×2.4 cylindrical-lenstelescope 50 to produce widening in the non-guided direction beforebeing focused onto a poled region of the PPLN waveguide device 70 ofFIG. 1 by a ×10 microscope objective 60. Such a combination of opticswas chosen to provide good launch efficiency whilst helping to reducedivergence in the horizontal unguided plane. This resulted in a pumpsource with a line focus and measured spot sizes of 4±1 μm in the guideddirection and 11±1 μm in the non-guided direction.

It should be noted that focusing to a waist in the non-guided plane atthe input face is not necessarily the optimum condition for maximum SHGefficiency. However, it was used in this demonstration due to thesimplicity of having one ×10 objective as the focusing element insteadof a more complicated cylindrical-lens launching arrangement. Also, forthis initial demonstration, both the input and output end-faces of thewaveguide were polished but left uncoated, leading to 14% reflectionlosses at each face.

The waveguide device 70 was placed in an oven 80 to maintain thewaveguide's temperature at the phase-matching temperature of 174.1° C.

A second ×10 microscope objective 90 was used to collect the transmittedlight from the waveguide. This was followed by an infra-red filter 100to block any throughput from the pump beams, allowing the generatedgreen output of the PPLN to be measured by an optical power meter. For204 mW of launched pump power (λ=1064 nm), a second-harmonic (SH) powerof 1.8 mW (λ=532 nm) was generated internal to the crystal. FIG. 3 showsa plot of the square root of the SH power versus launched pump power,revealing a quadratic dependence between the measured values.

It should of course be noted that the system of FIG. 2 is a specificexample of an optical parametric device. The waveguide would be suitablefor use in many other such devices.

Due to the unusual pumping geometry used while testing the PPLNwaveguide, any calculation of the SHG efficiency from the device wouldbe complicated. Instead, the most interesting comparison to make is witha calculation of the SH power expected from a similar length of bulkPPLN with optimised focusing in the centre of the grating. Assuming anon-linear coefficient of 16 pm V⁻¹ (a value consistent with results inbulk experiments using similarly produced PPLN gratings), it is possibleto produce a SH output power of 1.3 mW in the bulk material—a lowerresult than the 1.8 mW obtained from the direct-bonded waveguide.Therefore, it would appear that even with non-optimum focusing and onlyone guided dimension, the buried PPLN device shows an improved SHGefficiency over the bulk material.

Characterisation of the output modes of the PPLN waveguide was performedby the use of a video camera and PC-based evaluation software.Surprisingly, it was observed that both the 1064 nm throughput and theSH generated 532 nm radiation from the PPLN waveguide were in thefundamental spatial mode, an unexpected result for a 12-μm-thick guidewith such a large index difference (Δn_(e)≈1%). Indeed, only by using adeliberately poor launch was it possible to excite anything other thanthe fundamental mode at 1064 nm. Even more unusual was the result thatthe 1064 nm throughput from the unpoled LiNbO₃ region within the sameburied structure was multi-spatial-mode in nature. This clear differencein the mode properties, combined with the apparently differenttransmissions described earlier, suggests that the index profile of thePPLN section is different to that of the unpoled LiNbO₃ section.

In summary, for the first embodiment of the invention these experimentsdemonstrate the successful prototype fabrication of a 12-μm-thick,5.5-mm-long, symmetrical PPLN waveguide buried in LiNbO₃ by DB. Usingthe 6.50-μm-period PPLN grating at an elevated temperature of 174° C.,an efficient quasi-phase-matched frequency doubling of the 1064 nm lineof a cw diode-pumped Nd:YAG laser has been demonstrated. For 204 mW offundamental pump power, nearly 2 mW of green power was generated at anoutput wavelength of 532 nm. This result was obtained with non-optimumfocusing conditions but remains higher than the theoretical expectationfor a similar length of bulk material. The waveguiding properties wereshown to be different in the PPLN, and unpoled LiNbO₃ regions of thesample, with the PPLN section showing a surprising single-spatial-modebehaviour. These results suggest that the production of longer buriedwaveguides, potentially incorporating channel structures, should lead tohighly-efficient non-linear devices. With a full characterisation ofpropagation losses and effects of strain upon the index profile, the DBtechnique should allow extra freedom, and hence new devicepossibilities, in the choice of non-linear waveguiding structures.

The techniques described above are not limited to PPLN, but can beapplied to any optically useful poled ferroelectric material such asLiTaO₃, doped LiNbO₃ (e.g. Mg-, Ti- or rare earth doped), strontiumbarium niobate, barium titanate, potassium titanyl phosphate and itsisomorphs, polar semiconductors such as gallium arsenide and so on.

The poling of the PPLN can be carried out before, during, between and/orafter the bonding stages. If the poling is carried out other than beforethe bonding stages, and a ferroelectric material is used for the otherlaminas, then those other laminas can also end up being poled. This maychange the guiding properties of the waveguide but does not preventoperation as a waveguide. Indeed, the bonding properties may even beimproved by this measure (or by poling the other laminas separately).

In the example above, a poled area is used to define a waveguiding pathalong the lamina 10, but with other substrates it may be found that anunpoled lamina offers a more appropriate path.

It is not necessary to surround the lamina 10 by two other laminas 20.Instead, one lamina 20 could be used, to form an “open sandwich”structure of just two laminas. In this case the symmetry of thestructure would be altered and the guided mode(s) would probably bedifferent, but operation as a waveguide would still be possible.

The thickness of the lamina 10 can be altered, again altering the natureof the guided mode(s) in the waveguide. In this way, a single modestructure can be fabricated.

Referring now to FIGS. 4 and 5, a second embodiment using an indiffusiontechnique to define a waveguide path will be described.

In this second embodiment, a piece of PPLN 100 is made by theconventional electrical poling method. The piece 100 might be, forexample, 500 μm thick and several mm in the other two dimensions. Oneface 110 of the piece 100 is patterned with magnesium oxide (using aprocess of photolithography and vacuum evaporation or sputtering). Themagnesium oxide lamina is less than about 400 nm thick, and defines (bythe parts not covered by the lamina) a waveguide path along the piece100. The piece 100 is then heated to a temperature of between about 600°C. and about 1200° C. This causes the magnesium oxide material todiffuse in and, in the indiffused regions 130, locally lower therefractive index.

The piece 100 is then bonded, by a direct bonding process applied to theface 110, to a LiTaO₃ substrate (140, FIG. 5), before being polisheddown to a substantially uniform thickness of between, say, about 4 μmand about 40 μm.

A further magnesium oxide pattern is then deposited on the exposed(newly polished) face of the piece 100, and the heat treatment repeated.This causes the magnesium oxide to indiffuse from the other side, tomatch the indiffusion from the face 110. Regions 150 of reducedrefractive index are thus formed, defining a waveguiding core 160.

This technique, or a complementary out-diffusion technique, isapplicable not only to other ferroelectric materials (for examples, seeabove), but also to any substrates whose refractive index can be alteredby an indiffusion technique, such as various glasses, polymers and othercrystals. The common advantage shared between all applications of thistechnique is that the guiding region 160 can be formed of unadulteratedmaterial.

Similarly, in all of the embodiments, the “superstructure” laminas canbe of various materials such as unpoled LiNbO₃ or other suitablematerials from the lists above.

It is possible to fabricate curved waveguide paths using the abovetechniques. In the case of the first embodiment, a poling patternsimilar to that shown schematically in FIG. 6 can be used, where aseries of poled regions form a track 170 which bifurcates as a signalsplitter. In the case of LiNbO₃ there is a preferred poling directionresulting form the crystal structure, but the arrangement of FIG. 6 getsaround this restriction to form curved or varying-direction paths usingmultiple displaced poled stripes.

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What is claimed is:
 1. An optical waveguide comprising: a guiding laminaof optical material bonded by direct interfacial bonding to asuperstructure lamina of optical material, and a second superstructurelamina bonded by direct interfacial bonding to the guiding lamina, theguiding lamina defining a light guiding path, wherein said path isformed of an unmodified optical region of the guiding lamina and amodified optical region defines a boundary of said path.
 2. A waveguideaccording to claim 1, in which the guiding lamina is formed of aferroelectric material.
 3. A waveguide according to claim 2, in whichthe guiding lamina is formed of lithium niobate.
 4. A waveguideaccording to claim 2, in which the modified regions are electricallypoled regions of the guiding lamina.
 5. A waveguide according to claim4, in which the modified regions are spatially periodically electricallypoled regions of the guiding lamina.
 6. A waveguide according to claim1, in which the modified regions are formed by indiffusion of one ormore dopant materials into the guiding lamina.
 7. A waveguide accordingto claim 1, in which at least part of the modified regions form thelight-guiding path.
 8. An optical parametric device comprising: awaveguide according to claim 1; and means for launching an input opticalsignal into the waveguide.
 9. A device according to claim 8, comprising:an output filter for filtering light emerging from the waveguide toreduce components having the wavelength of the input optical signal. 10.A method of fabricating an optical waveguide, the method comprising thesteps of: (a) bonding, by direct interfacial bonding, a guiding lamina(10) of optical material to a superstructure lamina of optical material;(b) modifying optical properties of regions of the guiding lamina so asto define a light guiding path along the guiding lamina; (c) removingmaterial from the guiding lamina to reduce the thickness of the guidinglamina; and (d) bonding, by direct interfacial bonding, a furthersuperstructure lamina to the guiding lamina.
 11. A method according toclaim 10, further comprising: before step (a), indiffusing and/orout-diffusing material to/from one face of the guiding lamina, that facebeing bonded to the superstructure lamina in step (a); and before step(d), indiffusing and/or out-diffusing material to/from the exposed faceof the guiding lamina, that face being bonded to the furthersuperstructure lamina in step (d).